Allergic patients have been increasing in number every year in many countries including Japan, and high incident of allergic adults, one out of three in Japan, is reported. Allergic diseases are categorized into four types, type I to IV, depending on their mechanism of action. Some kinds of allergic rhinitis such as pollinosis, bronchial asthma, and atopic dermatitis are Type I immunoglobulin E (IgE)-mediated allergy, wherein increase in antigen-specific IgE level in blood enhances the risk of developing allergic symptoms.
The mechanism of development of Type I allergy is as follows. When an antigen, such as pollens, house dust, or mites, invades the body, an IgE antibody specific to such antigen is produced, and binds to mast cells or Fcε receptors on the basophil surface to sensitize the subject. When the antigen further invades the body, the antigen binds to the IgE antibody to form a complex. This causes degranulation, wherein chemical mediators in the granules, such as histamine and leukotoriene, are released to develop allergic symptoms.
Recently, allergic diseases are treated mainly with antagonists to chemical mediators, such as antihistamine, and steroids used as anti-inflammatory agents. However, both of these agents merely provide symptomatic therapy, and steroids inhibit the overall immune response, resulting in side effects. Alternatively, agents for inhibiting release of chemical mediators by inhibition of degranulation are also used, but no fundamental therapeutic agents have not been found for specifically reducing the IgE antibody, which is the major factor of allergy development.
Further, for necessary chronical administration, antiallergic agents that are easy to take and highly safe are desired. Accordingly, novel antiallergic agents having such properties are demanded.